# Power Electronics (Test 3)

## Gate Exam : Ee Electrical Engineering

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Q.1
In the figure shown below, the chopper feeds a resistive load from a battery source. MOSFET Q is switched at 250 kHz, with a duty ratio of 0.4. All elements of the circuit are assumed to be ideal The average source current in Amps in steady-state is
A. 3/2
B. 5/3
C. 5/2
D. 15/4
Explaination / Solution:
No Explaination.

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Q.2
The transistor circuit shown uses a silicon transistor with VBE = 0.7V, IC ≈ IE and a dc current gain of 100. The value of V0 is A. 4.65V
B. 5V
C. 6.3V
D. 7.23V
Explaination / Solution:
No Explaination.

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Q.3
The Voltage Source Inverter (VSI) shown in the figure below is switched to provide a 50 Hz, square-wave ac output voltage (v0) across an R-L load. Reference polarity of v0 and reference direction of the output current i0 are indicated in the figure. It is given that R = 3 Ω, L = 9.55 mH. In the interval when v0 < 0 and i0 > 0 the pair of devices which conducts the load current is
A. B. C. D. Explaination / Solution:
No Explaination.

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Q.4
The Voltage Source Inverter (VSI) shown in the figure below is switched to provide a 50 Hz, square-wave ac output voltage (v0) across an R-L load. Reference polarity of v0 and reference direction of the output current i0 are indicated in the figure. It is given that R = 3 Ω, L = 9.55 mH. Appropriate transition i.e., Zero Voltage Switching (ZVS) / Zero Current Switching (ZCS) of the IGBTs during turn-on / turn-off is
A. ZVS during turn off
B. ZVS during turn-on
C. ZCS during turn off
D. ZCS during turn-on
Explaination / Solution:
No Explaination.

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Q.5 A. B. C. D. Explaination / Solution:

When diode is ON, i=1mA. When diode is OFF, i is zero after small reverse recovery time.

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Q.6
An SCR is considered to be a semi-controlled device because
A. it can be turned OFF but not ON with a gate pulse
B. it conducts only during one half-cycle of an alternating current wave
C. it can be turned ON but not OFF with a gate pulse
D. it can be turned ON only during one half-cycle of an alternating voltage wave
Explaination / Solution:
No Explaination.

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Q.7
Assuming the diodes to be ideal in the figure, for the output to be clipped, the input voltage vi must be outside the range A. −1V to − 2V
B. −2V to − 4V
C. +1V to − 2V
D. +2V to − 4V
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Q.8
In the figure shown, all elements used are ideal. For time t<0, S1 remained closed and S2 open. At t = 0, S1 is opened and S2 is closed. If the voltage Vc2 across the capacitor C2 at t = 0 is zero, the voltage across the capacitor combination at t=0+ will be A. 1V
B. 2V
C. 1.5 V
D. 3 V
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Q.9
Transformer and emitter follower can both be used for impedance matching at the output of an audio amplifier. The basic relationship between the input power Pin and output power Pout in both the cases is
A. Pin = Pout for both transformer and emitter follower
B. Pin > Pout for both transformer and emitter follower
C. Pin < Pout for transformer and Pin = Pout for emitter follower
D. Pin = Pout for transformer and Pin < Pout for emitter follower
Explaination / Solution:
No Explaination.

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Q.10
A single-phase SCR based ac regulator is feeding power to a load consisting of 5Ω resistance and 16 mH inductance. The input supply is 230 V, 50 Hz ac. The maximum firing angle at which the voltage across the device becomes zero all throughout and the rms value of current through SCR, under this operating condition, are
A. B. C. D.   